It is known to equip motor vehicles with a level-adjusting device, for example in order to increase the ground clearance if necessary. Typically, although not necessarily, this adjusting device engages in one of the foot points of the spring of the wheel suspension in question and changes it, but in principle it is not decisive whether the spring foot point allocated to the base structure or else that allocated to the wheel in question is adjusted. Besides pneumatic and electrical spring foot point adjusting drives (see, for example, DE 102007051971 B4), especially hydraulic spring foot point adjusting drives are also known, for example from DE 3223195 A1, DE 102009047100 A1, U.S. Pat. No. 5,181,696 A, EP 2301773 A1, JP 2010-149550 A and WO 2014/142160 A1.
Several of the hydraulic level-adjusting systems are based on the approach (which is entirely logical) that only raising of the base structure of the vehicle takes place actively, i.e. by hydraulic energy, whereas the vehicle's own weight is used for lowering the bottom structure, in order to force the hydraulic fluid from the working space of a linear actuator (which accordingly is of only single-acting construction) via an opened return-flow line back into the reservoir. This is the case in particular for the system according to WO 2014/142160 A1, which discloses motor-vehicle chassis of the type mentioned in the introduction that has been particularly highly developed in this respect. Therein the hydraulic level-adjusting device comprises a single-acting hydraulic linear actuator, the working space (which can be pressurized in order to raise the base structure) of which is in communication with a (first) pump port of the pump of a hydraulic power pack via a line and valve arrangement. In order to hold the base structure of the motor vehicle reliably in its raised position without energy consumption, a shutoff valve, which has a valve piston biased by means of a closing spring in a shut-off position of the valve, is provided between this hydraulic power pack and the linear actuator. By pressurizing a valve working space allocated to it, the valve piston can be displaced against the force of the closing spring and in this way the shutoff valve can be opened, in order to drain the hydraulic fluid from the linear actuator and in this way to lower the base structure of the chassis. The hydraulic power pack is reversible, so that it can be switched between two delivery directions. The line and valve arrangement places the second pump port in communication with the valve working space of the shutoff valve and in addition comprises a 3/3-way valve placing the two pump ports in communication with one another, while its third port is in communication with the reservoir. The 3/3-way valve is biased by spring-loading in a shut-off position, from which, when actuated as a function of control pressure, it connects either the first or the second pump port to the reservoir.
In practice, however, the motor-vehicle chassis disclosed in WO 2014/142160 A1 and equipped with a hydraulic level-adjusting device does not achieve satisfactory results, despite the considerable equipment complexity. For example, only partial lowering of the chassis from its raised position is not possible or is possible only in complex manner; for example, to enable the return flow of hydraulic fluid from the working space of the linear actuator into the reservoir by means of the hydraulically opened shutoff valve, the piston of the shutoff valve is displaced—by pressurizing the working space in question of the shutoff valve for reversed delivery direction of the pump—against the force of the holding spring, and so the shutoff valve remains open until the delivery direction of the pump is changed once again, i.e. until the pump is again being operated with its first delivery direction and—with simultaneous pressurization of the working space of the linear actuator and hereby raising of the base structure of the chassis—the pilot-to-open check valve connected between the second pump outlet and the valve working space of the shutoff valve opens.
In view of the disadvantages of the prior art shown in the foregoing, an object of the present invention is to provide a motor-vehicle chassis of the type mentioned in the introduction that is characterized by improved practical utility, specifically while also reducing the equipment complexity compared with the prior art.